1. Field of Use
This invention relates generally to a fail-safe springloaded hydraulic brake and to a control system therefor. In particular, it relates to a brake and control system employed to slow, arrest or prevent rotation of a rotatable component in a machine.
2. Description of the Prior Art
Some machines, such as mobile cranes, employ a speed reducer having rotatable input and output shafts to transmit motive power between the drive shaft of a motor and a rotatable component on the machine, such as the horizontally rotatable or swingable upper section of the crane. The speed reducer provides necessary speed reduction and increased mechanical advantages. In some instances it is necessary to immediately arrest the motion of the movable component as soon as the motor is stopped (either intentionally or because of a system failure) or to slow or momemtarily stop the motion while the motor is still running. It is also necessary in some cases to prevent unintentional motion of the component whenever the motor is stopped, either during or at the end of operation of the machine. Accordingly, a suitable brake is provided for such purposes and such brake can act directly on the motor shaft or on the shafts of the speed reducer or on the movable component.
U.S. Pat. No. 4,263,991 issued Apr. 28, 1981, to Morgan et al. discloses a hydraulically operated brake which is connected between the output shaft of a hydraulic motor and the input shaft of a speed reducer. The brake comprises a stationary housing, and a motor-driven rotor shaft journaled in the housing. A number of rotor discs are carried on the shaft for rotation therewith, but they are adapted for axial sliding motion on the shaft, as by a spline arrangement. A number of stator discs are interleaved with the rotor discs and are carried on torque pins mounted in the housing for sliding motion in the direction of the rotor shaft axis, but are secured against rotational movement with the rotor shaft and rotor discs. Biasing springs urge the stator discs into braking engagement with the rotor discs. To relieve this braking action and permit free shaft rotation, a primary piston, having an annular skirt surrounding the rotor discs, engages one stator disc to apply a force urging that stator disc in a direction opposite to the force applied by the biasing means so as to release the braking action. A secondary device is included for opposing the primary piston action so as to permit the biasing means to urge the stator discs into braking engagement with the rotor discs. This secondary device can be independently energized, as by fluid pressure or mechanical means, as by a service brake. This secondary device includes a piston carried in an annular cylinder chamber and the piston is located to apply a force against the stator disc in opposition to the force applied by the primary piston.